1. Field of the Invention
This invention relates to a disk, a disk device and a track center detection method. More particularly, the present invention relates a disk, a disk device and a track center detection method adapted to define and detect a track center.
2. Description of the Related Art
DVD-RAMs are among typical products of the recently developed technologies of digital versatile disks (DVD). A DVD-RAM is provided with addresses realized in the form of pre-pits that are allocated to the sectors thereof on a one by one basis. The addresses are referred to as complementary allocated pit addresses (CAPAs). With a system using CAPAs, the center of the track of pre-pit header fields and the center of the track of fields where user data is recorded are displaced in opposite directions by a half of the pitch of the track arrangement.
The amplitude of each of the signals reproduced from two groups of header fields (the amplitude of the signal of the sum of the quantities of reflected light beams) is used for detecting the center of the track thereof. The two groups of header fields are displaced in opposite directions by a half of the pitch of the track arrangement relative to the center of the track of the recording fields. The center signal representing the center of the track is extracted from the amplitudes of the reproduced signals.
More specifically, a track center signal is generated on the basis of the difference between the amplitude of the VFO (variable frequency oscillator) signal in a group of header fields displaced to a side from the track center by a half of the pitch of the track arrangement and that of the VFO signal in a corresponding group of header fields displaced to the other side from the track center by a half of the pitch of the track arrangement. With this method, if the signal reproducing beam traces the center of the track of the recording field, the amplitudes of the two reproduced VFO signals are substantially equal to each other because the two groups of head fields are displaced in opposite directions by the same distance. Then, the difference, or the value of the track center signal, is substantially equal to 0.
When the pre-pits of a pair of groups of header fields are arranged symmetrically at the opposite sides, as in the case of DVD-RAMs, a center signal is generated by using the difference of the amplitudes of the two reproduced signals as described above.
However, from the viewpoint of the efficiency of simply reproducing the signals from the header fields, it is highly efficient when the signal reproducing light beam traces exactly above the pre-pits. It is highly efficient too, from the viewpoint of stability of the operation of signal reproduction, because the offset margin of the two amplitudes is large.